This invention relates to a process for the production of alpha alumina and more specifically to a process for the production of alpha alumina from a precursor using a dry process.
Alumina is one of the most important and widely used abrasive materials. Some years ago the industry standard was artificially produced corundum, or alpha alumina, made by a thermal process in which bauxite was converted to the oxide. After the invention of the sol gel process in which a sol of boehmite was gelled, dried and then fired to convert to alpha, it was realized that the finer the crystal size in the abrasive grains, the better the abrasive performed. This led to the seeded sol gel process of Cottringer et al. (U.S. Pat. No. 4,623,364), which produces sub-micron sized crystallites by introducing sub-micron sized seed particles of alpha alumina or equivalent seed material into a boehmite sol which is then gelled, dried and fired as before. This was found also to decrease the temperature at which conversion to alpha alumina occurs. However sol-gel processes all have the disadvantage that it is necessary to use large volumes of water which have to be heated, transported and eventually removed at some time during the process. Elimination or reduction of this water could, in some situations, be very desirable.
It has been recognized for some time that an alpha alumina precursor, such as gamma alumina, can be induced to undergo conversion to the alpha phase at a comparatively low temperature under the influence of a dry milling operation. This effect is described in a paper by Lin, Nadiv and Baron, (Thermochimica Acta, 148 (1989), pp. 301-310, which describes milling gamma alumina at room temperature and thereafter heating to 800.degree. C. to convert to alpha alumina. The conversion to alpha alumina was measured using X-ray diffraction techniques. It was found that the longer the gamma alumina had been milled, the greater the proportion of the gamma was converted to alpha even at this temperature, which is some 400.degree. C. below the normal conversion temperature.
In addition, Panis, in a series of articles in C.R. Acad. Sc. Paris, vol. 271, No. 13, Series D, pages 153-155, 945-948 and 1057-1059, disclosed that, after dry grinding at elevated temperatures at atmospheric pressure, both gamma alumina and boehmite could be transformed essentially completely into alpha alumina if the grinding were continued long enough. In addition it was found that grinding reduced the conversion temperature to alpha alumina. It is not however practical to grind in volume at such temperatures so that these findings have not roused much commercial interest. Panis also taught that boehmite can be dry milled at room temperature to reduce the temperature at which it transforms to the alpha form.
It has been speculated, for example by Dynys and Halloran, (J. Amer. Ceram. Soc. Vol. 65, No. 9 pp 442-448), that the dry milling causes a mechanochemical transformation, that is, a chemical transformation induced in a dry material by the application of mechanical forces, and is similar to other reported accelerations of phase transformation by mechanical action during milling.
It has now been found possible to obtain a significant conversion of boehmite or gamma alumina to alpha alumina in a room temperature operation by operating under specific conditions. This offers the opportunity of a very attractive route to alpha alumina, perhaps starting from the readily available and sol-gel processable boehmite, that has great flexibility of application and potential for significant savings.
The present invention also provides a process that significantly reduces the temperature of thermal conversion, (as measured by differential thermal analysis), of an alpha alumina precursor to alpha alumina. It also provides a process for treating an alpha alumina precursor to yield a product that would sinter to alpha alumina with at least 97% of theoretical density, at a temperature that is at least 50.degree. C. below the sintering temperature of the untreated precursor.